1. Field of the Invention
This invention relates generally to pressure sensors and, more specifically, to a Pressure Sensing Apparatus.
2. Description of Related Art
The bellows-type pressure sensor is widely used in high sensitivity applications. Essentially, what is involved is a very small bellow that is configured to reflect light onto a detector. When the bellow stretches or contracts in response to a pressure change, the detector will sense a corresponding change in the light intensity.
FIG. 1 is a perspective view of a prior art bellow assembly 10. As can be seen, from its outer dimensions, the bellow assembly 10 comprises a stem 12 from which extends the bellow section 14 which terminates in the head 16. The bellow section 14 is formed somewhat like an accordion, such that the bellow assembly 10 can stretch and shrink in response to changes in external forces. If we now turn to FIG. 2, we can examine the details of how this bellow assembly 10 functions to detect pressure.
FIG. 2 is a partial cutaway side view of the bellow assembly 10 of FIG. 1. Again we can see that the bellow section 14 extends from the stem 12 and terminates in the head 16. In this current embodiment the head 16 comprises a reflector 18 formed on its inner surface. Within the bellow section 14 is conventionally located a light detector 20 mounted on a stand 22. Also found within the bellow section 14 is a light emitter 24. The light emitter is configured to transmit light to the reflector 18 where it is in turn reflected towards the light detector 20. In a conventional bellow assembly pressure detector 10, the light detector 20 is sensitive enough to detect a change in light intensity in response to a change in bellow length 26. It should be noticed that in this conventional design, the reflector 18 has always been substantially flat. As such, the reflective light does not converge in any sort of focal point but instead essentially reflects outward in a Gaussean distribution and is spread into a wide area at the depth of the receiver 20; when the reflector moves towards the receiver 20, reflected areas become smaller (in effect focusing the signal). If one imagines that the bellow assembly 10 has an internal pressure 1 (which may be effectively zero) and the bellow assembly 10 is located within another volume at a unknown pressure PX, as PX is changed, the bellow length 26 will also change to some length determined by the pressure difference and the physical properties of the bellow. It is this bellow length change 26 that is detected by the detector 20 and converted into an electrical signal for display to the user. FIG. 3 depicts further information about this prior art device.
FIG. 3 is a partial cutaway side view of a conventional bellow-type pressure sensor 30 of the present invention. As can be seen, bellow assembly 10 is typically located within a chamber 28. If we imagine that the bellow assembly 10 is isolated from the chamber 28 and that the chamber 28 includes a sensor tube 32 for sensing an external pressure, we can appreciate that when the sensor tube 32 is placed in a location such that the pressure PX changes from some reference pressure, and the bellows 10 later extend or contract while the internal pressure P1 seeks to reach equilibrium with the sensed or unknown pressure PX. If we now turn to FIGS. 4A through 4C we can discuss the operation of the prior device more fully.
FIG. 4A is a depiction of the signal path of the bellow assembly 10 of FIGS. 1, 2 and 3. In this simplified drawing, the reflector 18 is shown at a distance LX1 from the detector 20. We will assume at this point that LX1 defines the at rest condition of the bellow 10. As can be seen, the transmitted light 34 from the transmitter (not shown) strikes the reflector 18 and is reflected back as reflected light 36. As discussed above, it should be understood that substantially all of the transmitted light 34 is returned along the identical path of its arrival 36. Some light however, will scatter as a result of surface irregularities on the reflector 18 and it is this light that is most likely received by the detector 20 (and therefore may contribute to the dynamic range). If we turn to FIG. 4B we can see that when the sensed pressure changes, the distance between the reflector 18 and the detector 20 changes to LX2. 
FIG. 4B is a depiction of the device of FIG. 4A after a pressure change has occurred. It should be casually apparent that the reflected light 36 is not substantially changed by the change in the location of the reflector 18. In fact, in order to sense this changed distance, detector 20 must be extremely sensitive (and therefore expensive). Even still, this design will provide a fairly responsive and sensitive pressure detector having a dynamic range in the area of 2 dB. If we now turn to FIG. 4C we can see yet another limitation of the prior sensor.
FIG. 4C is a depiction of the device of FIGS. 4A and 4B when the device is experiencing off-axis deflection. As can well be imagined, the bellow 10 in order to be sensitive, is formed from very thin-walled material. As such, it is affected by external forces including vibrations, gravity and other acceleration and it is common for these external forces to result in an off-axis deflection θy. As can be seen here, while the transmitted light 34 has not changed, when a theoretical deflection θy is caused in the bellow 10, the reflected light 36 tends to be directed away from the detector 20. As such, where the sensor is experiencing vibrations they might actually be sensed as pressure changes but in fact this is not necessarily the case. This, again, adds expense because the detector must be isolated for many external acceleration-type forces.
What is needed therefore, is an improved pressure sensor that will increase responsiveness of the detector while reducing the need for an extremely sensitive detector. It would further be desirable if the improved sensor was less sensitive to off-axis deflection.